This application claims priority over Japanese Patent Application Ser. No. 115366/77 filed Sept. 26, 1977.
This invention pertains to the molding of silicone rubber tubes from heat-vulcanized silicone rubber composition by using the processes of extrusion and hot air vulcanization. In particular, it is a method which significantly improves the condition of the silicone rubber tube surface by processing the silicone rubber compound composed of the above-mentioned materials through extrusion, hot air vulcanization, and the application of appropriate post vulcanization as necessary. The term "hot air vulcanization" in this specification refers to the process wherein a non-vulcanized rubber compound is passed through hot air for a short time in a vulcanizing furnace heated to more than 200.degree. C.; similarly, the term "post-vulcanization" refers to the process whereby the product which was vulcanized by the hot-air method or any other method is exposed to hot air to complete its vulcanization.
Conventionally, either of the following methods is used in the manufacturing of silicone rubber tubes by hot-air vulcanization.
The first method calls for the mixing of the silicone rubber compound composed of silica powder and silicone rubber gum which is a high-polymer methylvinylpolysiloxane, with an aromatic acylorganicperoxide such as 2,4-dichlorobenzoylperoxide in catalytic amounts as the vulcanizing agent, followed by its extrusion by an extruder into a furnace heated to 200.degree.-400.degree. C. to perform hot-air vulcanization. In this instance, sometimes the compound is not extruded directly into the heating furnace by the extruder, but is preheated by a burner or heater.
The second method calls for the mixing of the silicone rubber compound composed in substance of silica fine powder and silicone rubber gum which is a high polymer methylvinylpolysiloxane with organohydrogenpolysiloxane as a bridging agent, and a platinum compound as a catalyst, followed by its extrusion into a furnace heated to 200.degree.-400.degree. C. by an extruder to perform its vulcanization.
In the first method, although there are no problems in roll-workability, scorching or extrusion; the tube surface remains adhesive after the process of hot-air vulcanization. This adhesion stays even after the silicone rubber tube produced by this method goes through a post-vulcanization treatment at a high temperature inside a dryer. In instances where silica fine powder is treated superficially with an organic silicon compound to produce a transparency of the silicone rubber tube, the problem of surface adhesion is particularly marked. The silicone rubber tube produced by the second method displays a superior surface finish with no adhesion problems. However, the method has its drawbacks in that the pot life and shelf life of the composition stability during the process and before vulcanization is not as good as would be desired. In addition, the composition is easily scorched.